1. Field of the Invention
This invention relates to an improvement in a process for the production of a methylene-crosslinked polyphenylene polyisocyanate. In particular, this invention is concerned with a process for the continuous production of a methylene-crosslinked polyphenylene polyisocyanate which does not contain as impurities acids and hydrolyzable chlorine-containing compounds at high concentrations and is not colored significantly. The term "acids" as used herein means acidic compounds formed upon reaction with an alcohol at room temperature. The term "hydrolyzable chlorine-containing compounds", on the other hand, means impurities which can be hydrolyzed at the boiling point of water to form acidic compounds. The acidic compounds so formed may hereinafter be designated by "HC".
Methylene-crosslinked polyphenylene polyisocyanates have extremely high reactivity and are used for the production of a wide variety of products such as polyurethane foams, elastomers, adhesives and paints.
2. Description of the Related Art
A methylene-crosslinked polyphenylene polyisocyanate (hereinafter abbreviated as "poly-MDI") is industrially produced by reacting a polyamine mixture, which has been formed by condensation of aniline and formaldehyde in the presence of an acid catalyst, with phosgene in the presence of a solvent. In general, diphenylmethane diisocyanate (hereinafter abbreviated as "MDI") is then removed by distillation under reduced pressure, thereby providing a poly-MDI of desired MDI content and viscosity. It is however known that the poly-MDI obtained by the above process contains the acid and HC as impurities and the inclusion of such impurities at high concentrations results in poor reactivity in the production of polyurethane foam.
Many methods have been proposed for lowering the acid and HC contents. Industrially, heat treatment at elevated temperature and reduced pressure is the simplest and most economical method. Poly-MDI is however prone to deterioration in hue during its treatment at elevated temperature or its heating for the removal of MDI. This deterioration in hue then causes coloration upon formation of a polyurethane foam. There is accordingly an outstanding desire for the development of a process which can produce poly-MDI which is not colored significantly and does not contain the acid and HC at high concentrations.
Regarding improvements in the hue of poly-MDI, Japanese Patent Laid-Open No. 58955/1985 discloses a representative method for the removal of color components from poly-MDI. According to this method, poly-MDI is extracted at 180.degree. C. or higher with an aliphatic hydrocarbon having 8 or more carbon atoms so that tar components are eliminated. This method however requires removal of the extraction solvent and treatment of the residual tar components and, accordingly, is not preferred for use in combination with industrial production processes.
Further, a degasification method making use of hydrogen chloride gas is disclosed in U.S. Pat. No. 4,193,932 (Japanese Patent Laid-Open No. 70220/1979). According to this method, hydrogen chloride gas is charged at elevated temperature while phosgene is still present in the mixture. This method however cannot sufficiently lower the acid and HC contents and improve the hue and, moreover, requires a long time. It is therefore not preferred for use in combination with continuous production processes.